Stator windings of large generators include conductor bars in the form of so-called Roebel bars (see, for example, U.S. Pat. No. 1,144,252, which is incorporated by reference herein). The conductor bars are exposed to the self-consistent magnetic field as well as to the main field of the machine. A subdivision into component conductors with a twist as is done with Roebel bars reduces the eddy current losses and the circulating current losses to an acceptable level.
In the 1970s, studies were carried out on air-gap windings for power limit generators with liquid-cooled or superconductive rotors. Here, the incentive existed to use Roebel bars with cabled stranded conductors (see, for example, Swiss publication CH 532 860 and Japanese publication JP-A 62100144).
These bars were consistently liquid-cooled. Here, the primary aim of the design was to achieve good thermal contact between the conductors and the cooling tube. It was likewise described that stranded cables were first press-formed into approximately rectangular component conductors, after which the solid copper component conductors were transposed (i.e. Roebel-transposed) to form the Roebel bar. The difficult process of cranking (FIG. 2 of U.S. Pat. No. 1,144,252), which is even more difficult for such pre-press-formed stranded conductors than for solid component conductors, is not described in these patent specifications.
It is also a known procedure (US-A 1-2002/0050395, incorporated by reference herein) to use rectangular stranded bars that have been integrally press-formed into windings and that consist of a plurality of insulated copper wires, without Roebel transposition, in small high-frequency alternators in automotive technology. Due to the absence of systematic position traveling along the entire cross section of the conductor, undesired circulating currents occur beyond a certain conductor size.
German utility model DE G8321135.1 describes Roebel-transposed stranded conductors for all kinds of coils. The conductors described can be shaped so as to have a rectangular cross section. Here, the conductors remain semi-finished products, that is to say, in a subsequent step, they can be wound to form coils, for example, in transformers. Consequently, no description is given of the compacting and permanent strengthening procedures.